Xenogeneic Collagen Matrix Versus Free Gingival Graft for Augmenting Peri‐Implant Keratinized Mucosa Around Dental Implants: A Systematic Review and Meta‐Analysis

ABSTRACT Objectives There is a growing evidence to suggest augmenting peri‐implant keratinized mucosa in the presence of ≤ 2 mm of keratinized mucosa. However, the most appropriate surgical technique and augmentation materials have yet to be defined. The aim of this systematic review and meta‐analyses was to evaluate the clinical and patient‐reported outcomes of augmenting keratinized mucosa around implants using free gingival graft (FGG) versus xenogeneic collagen matrix (XCM) before commencing prosthetic implant treatment. Material and Methods Electronic databases were searched to identify observational studies comparing implant sites augmented with FGG to those augmented with XCM. The risk of bias was assessed using the Cochrane Collaboration's Risk of Bias tool. Results Six studies with 174 participants were included in the present review. Of these, 87 participants had FGG, whereas the remaining participants had XCM. At 6 months, sites augmented with FGG were associated with less changes in the gained width of peri‐implant keratinized mucosa compared to those augmented with XCM (mean difference 1.06; 95% confidence interval −0.01 to 2.13; p = 0.05). The difference, however, was marginally significant. The difference between the two groups in changes in thickness of peri‐implant keratinized mucosa at 6 months was statistically significantly in favor of FGG. On the other hand, XCM had significantly shorter surgical time, lower postoperative pain score, and higher color match compared to FGG. Conclusions Within the limitation of this review, the augmentation of keratinized mucosa using FGG before the placement of the final prosthesis may have short‐term positive effects on soft tissue thickness. XCM might be considered in aesthetically demanding implant sites and where patient comfort or shorter surgical time is a priority. The evidence support, however, is of low to moderate certainty; therefore, further studies are needed to support the findings of the present review.


| Introduction
The influence of peri-implant keratinized mucosa on periimplant tissue health has been a subject of debate within the implant community for decades.Early reports, mainly on machined surface implants, failed to demonstrate a correlation between the amount of keratinized tissues and peri-implant soft tissue health or changes in marginal bone levels (Adell et al. 1985, 1986, Bengazi, Wennström, and Lekholm 1996, Lekholm et al. 1996, Wennström, Bengazi, and Lekholm 1994).Hence, a requirement for a minimum width or thickness of keratinized tissues around dental implants has not been established.In recent years, however, a growing body of evidence seems to shift our understanding with findings indicating a strong relationship between the lack of adequate keratinized mucosa and a significant accumulation of biofilm, peri-implant mucosal inflammation, and recession (Brito et al. 2014, Chung et al. 2006, Grischke et al. 2019, Lin, Chan, and Wang 2013, Perussolo et al. 2018, Schrott et al. 2009).In fact, in the presence of ≤ 2 mm of keratinized mucosa, augmentation procedures were deemed necessary for the maintenance of peri-implant tissue health (Giannobile et al. 2018, Sanz et al. 2022, Thoma et al. 2021).Moreover, Roccuzzo, Grasso and Dalmasso (2016) recommended the augmentation of keratinized mucosa in cases of ongoing periimplant mucosal recession or when patients are experiencing difficulty in maintaining adequate plaque control.Augmentation of keratinized mucosa has also been suggested to improve bleeding and plaque indices post peri-implant mucositis treatment (Basegmez et al. 2012).In these instances, augmentation with autogenous connective tissue grafts remains the gold standard, with some suggesting xenogeneic-derived substitutes as alternatives, albeit, with limited evidence.
While there seems to be a general consensus on the need for augmenting keratinized mucosa when insufficient, the appropriate surgical technique or timing of augmentation has yet to be defined.For example, an apically positioned flap has traditionally been used for the augmentation of keratinized mucosa (Friedman 1957).However, the relapse and contraction of the flap (Hillerup 1980) required the addition of autogenous tissue graft to enhance predictability and stability of the augmented mucosa (Basegmez et al. 2012, Hillerup 1980).Such grafts mandated a second surgical site with added postoperative morbidity to patients.Additionally, adequate size of these grafts might not always be available due to anatomical boundaries and their "patch-like" appearance might not be aesthetically pleasing (Basegmez et al. 2012, Reiser et al. 1996).Therefore, substitutes, such as collagen matrix, have been utilized to overcome these limitations.Collagen matrices are often xenogeneic and consist of two layers, a compact one for wound protection and a porous one for enhancing vascularization and clot stabilization (Ghanaati et al. 2011).Histological studies (Ghanaati et al. 2011, Vignoletti et al. 2011) showed that those matrices can integrate well with the surrounding tissues without any significant inflammatory response.Moreover, soft tissue substitutes were considered safe alternatives that meet the aesthetic expectations of patients and operators (Schmitt et al. 2016).Nevertheless, systematic reviews (Atieh et al. 2016, Huang et al. 2019) on the use of collagen matrices in the treatment of gingival recessions did not demonstrate significant differences between soft tissue substitutes and autogenous tissue grafts in terms of root coverage or gain in keratinized tissue with limited evidence to suggest improved postoperative morbidity or operating time.
Soft tissue augmentation around dental implants can be performed before implant placement, at the time of implant placement, before abutment connection, or after the placement of the final prosthesis.However, a general consensus on the ideal timing for augmenting the peri-implant keratinized mucosa remains lacking.Nevertheless, there is more emphasis on optimizing the peri-implant soft tissues before the prosthetic phase of implant treatment to improve future aesthetic outcomes and minimize any biological complications (Lin et al. 2018).Moreover, the outcomes of any augmentation procedure following the insertion of the final prosthesis were shown to be less predictable and were often seen as "rescue" procedures that required advanced surgical skills (Thoma, Muhlemann, andJung 2014, Thoma et al. 2014).Therefore, the current argument does not revolve around the necessity of a sufficient amount of peri-implant keratinized mucosa to maintain healthy and stable peri-implant tissues before completing implant treatment.Instead, the focus is on how we can predictably execute the augmentation of peri-implant keratinized mucosa before the insertion of the final prosthesis.Numerous clinical studies (Sanz et al. 2009, Schmitt et al. 2016, 2013, Solonko et al. 2022) showed comparable clinical outcomes in terms of gain in width of keratinized mucosa and aesthetic outcomes using different autogenous soft tissue grafts and substitutes.The conclusions, albeit were not decisive, particularly when comparing conventional surgical approaches utilizing a particular autogenous tissue graft or soft tissue substitute and performed at a specific time point during the rehabilitation.Hence, the aim of the present systematic review was to investigate the clinical and patient-reported outcomes of augmenting keratinized mucosa around implants using free gingival graft (FGG) versus xenogeneic collagen matrix (XCM) before commencing prosthetic implant treatment.

| Selection of Studies
Two reviewers (M.A.A. and N.H.M.A.) independently and in duplicate examined the retrieved citations on the basis of the title, abstract, and keywords.Irrelevant papers were excluded, and the full texts of the remaining ones were obtained.An eligibility form was used to examine papers for inclusion in the review.Any disagreements were resolved by discussion to reach a consensus or by consultation with a third reviewer (M.S.).In the event of duplicate papers, the one with the most relevant and sufficient information was selected.All the reasons for exclusion were reported.6) Length of the observation period.Any disagreements between reviewers were resolved by discussion to reach a consensus or by consultation with a third reviewer (M.S.).Corresponding authors were contacted for additional information if required.

| Quality Assessment of Included Studies
Two reviewers (M.A.A. and N.H.M.A.) used the Cochrane Collaboration's Risk of Bias tool for randomized trials (RoB 2) and the Risk Of Bias In Non-randomized Studies of Interventions (ROBINS-I) tool (Higgins et al. 2022, Sterne et al. 2016) to assess all the included studies independently and in duplicate.

| Data Synthesis
A statistical software program (Review Manager [RevMan] software, version 5.3, The Nordic Cochrane Centre, The Cochrane Collaboration, Copenhagen, Denmark) was used to conduct meta-analyses for studies of similar comparisons reporting the same outcome measures.For example, continuous data, such as changes in width of keratinized mucosa, were expressed in mean difference (MD) or standardized mean difference (SMD) and 95% confidence intervals (CIs).Randomeffects model was used to pool the results from more than one study as heterogeneity between studies was expected.With fewer than 10 studies, publication bias was not formally assessed because the power to detect publication bias was limited (Higgins et al. 2022).The statistical heterogeneity across different studies was assessed by means of Cochran's test for heterogeneity and I 2 statistic (Higgins et al. 2022).An I 2 value of > 50 indicated a substantial heterogeneity.The participant was considered as the statistical unit of analysis.To deal with within-patient correlation of clinical outcomes, we set the within-patient correlation coefficient of 0.9 and inflated the standard error where studies did not account for within-patient correlation (Higgins et al. 2022).In dealing with within-patient correlation, we followed the methods for dealing with clustered trials suggested in chapter 23 of the Cochrane Handbook (Higgins et al. 2022).
A leave-one study-out sensitivity analysis was conducted to check the source of heterogeneity, stability of results and influence of studies.Sensitivity analysis was conducted to assess whether estimated effects differ when we exclude studies at high risk of bias from analyses.The certainty of evidence was assessed using the five GRADE criteria (risk of bias, inconsistency, imprecision, indirectness, and publication bias) (Higgins et al. 2022)

| Characteristics of the Study Settings
A total of 101 studies were retrieved from the databases (Figure 1).After titles and abstracts were examined independently and in duplicate by two review authors (M.A.A. and N.H.M.A.), 10 studies were eligible for full-text review (Fu et al. 2021, Huang et al. 2021, Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Oh et al. 2017, Preidl et al. 2021, Qiu et al. 2023, Schmitt et al. 2016, Vellis, Kutkut, and Al-Sabbagh 2019, Tarasenko et al. 2020).Four studies (Fu et al. 2021, Oh et al. 2017, Preidl et al. 2021, Vellis, Kutkut, and Al-Sabbagh 2019) were subsequently excluded and as a result six studies (Huang et al. 2021, Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Qiu et al. 2023, Schmitt et al. 2016, Tarasenko et al. 2020) were included in the present review (Table 2).The main reason for exclusion was the timing of soft tissue augmentation, which was performed either before implant
3. At least one implant site that had shallow vestibule and ≤ 2 mm of keratinized tissue and required soft tissue augmentation before commencing restorative treatment (Huang et al. 2021, Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Qiu et al. 2023, Schmitt et al. 2016, Tarasenko et al. 2020).
The exclusion criteria were as follows: 1. Systemic conditions and/or medications that may interfere with healing (Huang et al. 2021, Lim, An, and Lee 2018, Qiu et al. 2023, Schmitt et al. 2016).

| Characteristics of the Interventions
All participants had implants that were planned for secondstage implant surgery.The augmentation of the keratinized mucosa was carried out either before (Qiu et al. 2023, Tarasenko et al. 2020) or at the time of uncovering the implants (Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Schmitt et al. 2016).The surgical procedure was relatively similar across the included studies.Before surgery, prophylactic antibiotics were administered (Qiu et al. 2023), and participants were asked to rinse with 0.12% chlorhexidine mouthwash for 30 s (Qiu et al. 2023) or 60 s (Lim, An, and Lee 2018, Tarasenko      Schmitt et al. 2016, Tarasenko et al. 2020).A partial thickness flap was raised and carefully dissected, then moved apically along the mucogingival junction, and fixed to the periosteum with interrupted sutures to create a new vestibule (Huang et al. 2021, Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Qiu et al. 2023, Schmitt et al. 2016, Tarasenko et al. 2020).In randomized controlled trials (Huang et al. 2021, Qiu et al. 2023, Tarasenko et al. 2020), the allocation to a specific treatment group was conducted following the preparation of the recipient bed.In the FGG group, the graft was harvested from the palate (Huang et

| Characteristics of Outcome Measures
The primary outcome measures were as follows: • Changes in width of peri-implant keratinized mucosa, as measured by a periodontal probe (Huang et al. 2021, Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Qiu et al. 2023, Schmitt et al. 2016, Tarasenko et al. 2020).
The secondary outcome measures were as follows: • Changes in thickness of peri-implant keratinized mucosa at 6 months, as measured by endodontic file (Huang et al. 2021) or cone beam computed tomography (Qiu et al. 2023).
• Changes in probing pocket depths, as measured by a periodontal probe (Huang et al. 2021, Qiu et al. 2023).
• Changes in modified sulcus bleeding index, as measured by a periodontal probe (Huang et al. 2021, Qiu et al. 2023).• Changes in modified plaque index, as measured by a periodontal probe (Qiu et al. 2023).
• Aesthetic outcomes (changes in color, texture, and contour), as assessed by using standardized clinical photos and scoring tools (Huang et al. 2021, Lim, An, and Lee 2018, Qiu et al. 2023).
• Operating time, as recorded by a digital timer (Huang et al. 2021, Schmitt et al. 2016).

| Risk of Bias in Randomized Controlled Trials
Three studies (Huang et al. 2021, Qiu et al. 2023, Tarasenko et al. 2020) were randomized controlled trials that were judged to be at low risk.They have adequately described the methods of randomization and allocation concealment, reported on masking the data assessors and showed no deviations from intended interventions (Table 3).

| Risk of Bias in Non-Randomized Studies
Overall, the remaining three studies (Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Schmitt et al. 2016) were judged to be at high risk (Figure 2).

| Bias due to Confounding
None of the studies showed any attempt to control for confounding.Nevertheless, none of the participants switched between interventions and baseline treatment was not influenced by prognostic variables.Therefore, all studies (Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Schmitt et al. 2016) were judged to be at moderate risk of bias.

| Bias in Classification of Intervention
All the studies (Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Schmitt et al. 2016) clearly defined the intervention and hence were judged to be at low risk of bias for this domain.

| Bias in Measurement of the Outcomes
The three studies (Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Schmitt et al. 2016) were rated at high risk of bias as none of those studies reported on masking the data assessors.

| Bias due to Deviation From Intended Interventions, Incomplete or Missing Outcome Data, or Selection of the Reported Results
None of the three studies (Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Schmitt et al. 2016) deviated from intended intervention, showed high risk of attrition, or bias due to selective reporting.Thus, they were rated at low risk.

| Sample Size Calculation
The three randomized controlled trials (Huang et al. 2021, Qiu et al. 2023, Tarasenko et al. 2020) reported on the sample size calculation.

| Clinical Trial Registration
Two studies (Huang et al. 2021, Qiu et al. 2023) were registered in the Chinese clinical trial registry before the initiation of the study.

| Effects of Interventions
In total, 174 participants were included in the present review.Of these, 87 participants had XCM while the remaining participants had FGG (Table 4).All the studies reported the data at the participant level.

| Aesthetic Outcomes
The changes in color and texture were reported in three studies (Huang et al. 2021, Lim, An, and Lee 2018, Qiu et al. 2023), while the changes in contour were reported in two studies (Huang et al. 2021, Lim, An, andLee 2018).The meta-analysis showed that implant sites augmented with XCM had more favorable changes in color when compared to implant sites augmented with FGG, but the difference was only marginally significant (MD −1.19; 95% CI −2.40 to 0.03; p = 0.06; Figure 4f).Likewise, the changes in texture (MD −0.62; 95% CI −1.76 to 0.52; p = 0.29; Figure 4g) and contour (MD −0.19; 95% CI −0.97 to 0.60; p = 0.64; Figure 4h) were in favor of XCM treatment group.However, the difference between the two treatment groups was not statistically significant.Substantial heterogeneity was noticed in the meta-analyses of the three aesthetic outcomes.

| Sensitivity Analyses
The leave-one study-out sensitivity analysis showed that the exclusion of one study (Lim, An, and Lee 2018), judged to be at high risk of bias, has influenced the overall effect size estimate Note: GRADE Working Group grades of evidence: High certainty: We are very confident that the true effect lies close to that of the estimate of the effect.Moderate certainty: We are moderately confident in the effect estimate: The true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different.Low certainty: Our confidence in the effect estimate is limited: The true effect may be substantially different from the estimate of the effect.Very low certainty: We have very little confidence in the effect estimate: The true effect is likely to be substantially different from the estimate of effect.Abbreviations: CI: confidence interval; FGG: free gingival graft; MD: mean difference; XCM: xenogeneic collagen matrix. a The risk in the intervention group (and its 95% CI) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI).
for changes in width of peri-implant keratinized mucosa at 6 months.The exclusion of this study showed that the difference between the two treatment groups was statistically significant in favor of FGG (Table 5).The meta-analysis of only randomized controlled trials, rated at low risk of bias, showed that sites augmented with FGG were associated with significantly less changes in the gained width of peri-implant keratinized mucosa compared to those augmented with XCM at 6 months (MD 1.48; 95% CI 0.24-2.72;p = 0.02).

| Summary of Main Results
The present systematic review compared implant sites augmented with FGG to those augmented with XCM, before commencing prosthetic implant treatment, in terms of changes in width of the keratinized mucosa, the thickness of keratinized mucosa, probing pocket depths, modified bleeding and plaque indices, aesthetic outcomes, patient-reported outcome measures, and operating time.Sites augmented with FGG showed less contraction compared with XCM, but the difference between the two groups reached a marginal statistical significance at 6 months.The difference between the two groups in terms of changes in thickness of keratinized mucosa at 6 months was statistically significant in favor of FGG.On the other hand, the use of XCM to augment keratinized mucosa was associated with significantly shorter operating time, significantly lesser postoperative pain and marginally significant improvement in color change.In terms of patient satisfaction and changes in contour and texture of augmented sites, the results were comparable between the two groups.

| Quality of Evidence
Three of the included studies in the present systematic review were not randomized and were rated at high risk of bias (Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Schmitt et al. 2016).The remaining three studies (Huang et al. 2021, Qiu et al. 2023, Tarasenko et al. 2020), however, were randomized and were judged at low risk of bias.A sensitivity analysis of the primary outcome was performed, and the removal of either one study (Lim, An, and Lee 2018) or the inclusion of only randomized controlled trials had an impact on the changes in width of keratinized mucosa at 6 months in favor of FGG.The overall certainty of evidence for the primary outcome looking at changes in width of keratinized mucosa at 1 month was judged moderate, whereas the evidence quality for the same outcome at 2-12 months was judged low.The quality of evidence for the remaining outcomes, including aesthetic outcomes and patientreported outcome measures, varied from moderate to low due to substantial statistical heterogeneity, lack of blinding, and imprecision.Despite the standardization in selection criteria of participants, surgical techniques, and timings of soft tissue augmentation, the inclusion of both anterior and posterior implant sites and methods of assessment (periodontal probe vs. cone beam computed tomography) could be regarded as potential sources of the observed heterogeneity.

| Applicability of Evidence
The present systematic review has shown that both FGG and XCM are effective techniques for augmenting the keratinized mucosa in implant sites with shallow vestibules and insufficient keratinized mucosa.Our findings showed that the gain in width and thickness of peri-implant keratinized mucosa was in accordance with other studies (Sanz et al. 2009, Schmitt et al. 2013, Solonko et al. 2022).XCM showed more shrinkage in width and thickness than FGG at different time points.Although the difference was not statistically significant at 1-3 or 12 months, it did reach a marginal statistical significance at 6 months.Moreover, the changes in thickness at 6 months were significantly more in the XCM group.The shrinkage of XCM has previously been reported in other studies (Nevins et al. 2011, Sanz et al. 2009), indicating that there is low to moderate evidence to suggest that FGG is more likely to maintain its volume up to 12 months as compared to XCM.It has been suggested that the shrinkage of XCM can be minimized by using a strip of FGG at the apical portion of the recipient bed that served as a mechanical barrier and cell source, maintaining the apical displacement of the desired mucogingival junction position, thereby generating the desired keratinized mucosa (Urban et al. 2015(Urban et al. , 2019)); however, the proposed procedure was outside the scope of the present review to assess.
Another vital aspect that contributes to maintenance of periimplant health and correction of aesthetic outcomes around implants is the thickness of the peri-implant keratinized mucosa.Although there is no consensus on the required thickness of peri-implant keratinized mucosa for optimal aesthetic and functional outcomes, a thick phenotype of ≥ 2 mm has been associated with lower incidence of mucosal recession and marginal bone loss (Tavelli et al. 2021, Thoma et al. 2018).Additionally, a thicker peri-implant soft tissue also aids in masking the "gray zone" of the implant-abutment interface.This meta-analysis suggests that XCM might not be an alternative to FGG, as FGG ensured greater stability of width and thickness of keratinized mucosa over 12-month observation time, aligning with prior findings on soft tissue augmentation around dental implants (Bassetti et al. 2016).Alternative procedures to increase the keratinized mucosal thickness around dental implants such as the bilaminar placement of subepithelial connective tissue graft or other substitutes with overlying pedicled graft have previously been described (Thoma et al. 2018).Their effectiveness, however, is outside the scope of this review.
XCM could be the preferred option in terms of patient perception of pain as pain score was significantly less as compared with the FGG group where a donor site is required (i.e., palate).This is in agreement with several studies (Schmitt et al. 2013, Solonko et al. 2022, Tonetti et al. 2018) in which the presence of a second surgical site increased postoperative pain and discomfort.As previously reported (Tavelli et al. 2021), the present review confirmed that the augmentation of peri-implant keratinized mucosa did not have any adverse events on periimplant health as assessed by probing pocket depths, bleeding, or plaque indices.The aesthetic outcomes were in favor of XCM, as XCM achieved a better match in color, texture, and contour when compared to FGG.The difference was only marginally significant in terms of color, but the overall aesthetic appearance of XCM grafted sites corroborated with other findings (Nevins et al. 2011, Sanz et al. 2009, Schmitt et al. 2013, Urban et al. 2015), which showed that XCM acts as a scaffold for oral keratinocytes that differentiate into matched keratinized mucosa.In contrast, FGG maintained its distinct color and texture, which is often described as a "tire-patch" appearance (Yukna et al. 1977).

| Agreements and Disagreements With Other Reviews
Despite the abundance of reviews on soft tissue augmentation around dental implants (Atieh and Alsabeeha 2020, Tavelli et al. 2021, Thoma et al. 2018, Zucchelli et al. 2020), only two (Bassetti et al. 2016, Lin et al. 2018) evaluated the impact of soft tissue augmentation at the time of uncovering dental implants or before the placement of the final prosthesis.The first review (Bassetti et al. 2016) was a systematic review that evaluated the impact of using different autogenous tissue grafts and substitutes on the dimensional changes of keratinized mucosa, aesthetics, and periodontal parameters.Meta-analysis was not attempted due to heterogeneity of the surgical techniques and augmentation materials used.The authors concluded that autogenous tissue grafts were still the gold standard in terms of long-term tissue stability.The second review of Lin et al. (2018) assessed the effect of timing of soft tissue augmentation on stability of width and thickness of keratinized mucosa.That review conducted metaanalyses on four studies that described augmentation of keratinized mucosa after implant placement and before placement of the final prosthesis.Two of the included four studies were from the same trial that compared FGG to XCM.The authors concluded that there were no differences between simultaneous or staged soft tissue augmentation in terms of changes in soft tissue width and thickness.Neither of the reviews made reference to patientreported outcome measures and did not provide any specific conclusions regarding the comparative effects of FGG versus XCM.
The present systematic review has several limitations that are mainly related to the retrospective and non-randomized design of some included studies and insufficient data on all the outcomes set for the review.Nevertheless, the meta-analysis, particularly when non-randomized controlled trials were excluded, confirmed the beneficial effects of FGG, as compared to XCM, in preserving the gain in width and thickness of keratinized mucosa.That positive impact, however, was on the expense of the aesthetics and perception of postoperative pain.It needs to be recognized that the existing literature on the influence of soft tissue augmentation before the placement of the final prosthesis is currently limited and additional long-term studies are still needed.

| Conclusions
Within the limitation of this review, the augmentation of keratinized mucosa using FGG before the placement of the final prosthesis may have short-term positive effects on soft tissue Two authors (M.A.A. and N.H.M.A.) used a data extraction form and independently collected the following information from the included studies: (1) Study characteristics: Title, authors' names, study location, language of publication, year of publication, published or unpublished data, source of study funding, and study design.(2) Participants: Demographic characteristics, inclusion/exclusion criteria, number of participants in test and control groups, attrition rate, and reasons for dropouts.(3) Interventions: Number of participants undergoing soft tissue augmentation with XCM.(4) Comparison: Number of participants undergoing soft tissue augmentation with FGG.(5) Outcomes: Changes in width and thickness of keratinized mucosa, changes in periodontal parameters, aesthetic outcomes, patient-reported outcome measures, and operating time.(

FIGURE 1 |
FIGURE 1 | Flowchart of the search process.

FIGURE 2 |
FIGURE 2 | Assessment of risk of bias of the included nonrandomized studies presented with low (green), moderate (yellow), and high (red) risk of bias.

FIGURE 4
FIGURE 4 Comparison: Xenogeneic collagen matrix versus free gingival graft.Secondary outcomes: (a) changes in thickness of keratinized mucosa at 6 months; (b) changes in probing pocket depth at 2-3 months; (c) changes in probing pocket depth at 6 months; (d) changes in modified sulcus bleeding index at 1-3 months; (e) changes in modified sulcus bleeding index at 6 months; (f) changes in color; (g) changes in texture; (h) changes in contour; (i) pain score; (j) satisfaction score; and (k) operating time.τ: Kendall tau; CI: confidence interval; FGG: free gingival graft; IV: inverse variance; SD: standard deviation; XCM: xenogeneic collagen matrix; z: z-test.

TABLE 1 |
Databases and search terms.

TABLE 2 |
Characteristics of the included studies.

TABLE 3 |
Assessment of risk of bias of the included randomized controlled trials.
(Lee, Kim, and Jang 2010, Lim, An, and Lee 2018, Schmitt et al. 20168, Schmitt et al. 2016) were judged to be at low risk of bias for this domain.

TABLE 4 |
Summary of findings.

TABLE 5 |
Leave-one study-out sensitivity analysis: Changes in width of peri-implant keratinized mucosa at 6 months.I 2 = 70% thickness.XCM might be considered in aesthetically demanding implant sites and where patient comfort or shorter surgical time is a priority.The evidence support, however, is of low to moderate certainty, and therefore, further studies are needed to support the findings of the present review.